A typical mobile communication network comprises a core network and a mobile access network often also referred to as radio (access) network. A mobile terminal can communicate via the mobile access network and the core network with other mobile terminals or fixed terminals or application servers.
Mobile communication technologies have been growing fast in history, however, the path of the evolution has not followed a monolithic and homogeneous technology trend. Thus, nowadays many different mobile communication technologies exist that are incompatible or at least complex to use in parallel from a user perspective. This obstacle has been recently addressed by mobile communication providers by introducing handover technologies enabling a transfer of a mobile terminal's access between mobile access networks having different access technologies.
The 3GPP (3rd Generation Partnership Project) System Architecture Evolution enables a mobile terminal to connect to the EPC (Evolved Packet Core) network via different 3GPP and non-3GPP access technologies and to perform handover between different access technologies, as described in 3GPP TS (Technical Specification) 23.401 and TS 23.402.
One way to optimize handover between 3GPP mobile access networks, for example GERAN (GSM EDGE Radio Access Network; GSM (Global System for Mobile communications); EDGE (Enhanced Data Rates for GSM Evolution)), UTRAN (UMTS Terrestrial Radio Access Network; UMTS (Universal Mobile Telecommunications System)), E-UTRAN (Evolved-UTRAN, wherein the E-UTRAN will also be referred to as ‘LTE’—Long Term Evolution—network hereinbelow) and WiMAX (Worldwide Interoperability for Microwave Access) access networks, has been proposed in 3GPP TSG SA WG2 Architecture, S2#61, S2-075215, 12-16 Nov. 2007, Ljubljana, Slovenia, BT et al. “Signalling flows for optimized handover between mobile WiMAX and 3GPP access (TS 23.402)”, Agenda Item: 8.4.3.2, SAE/Rel-8 and in 3GPP TSG SA WG2 Architecture, S2#61, S2-075605, 12-16 Nov. 2007, revision of S2-075214, Ljubljana, Slovenia, BT et al., “Architecture for Optimized Handovers between mobile WiMAX access and 3GPP access (TS 23.402)”, Agenda Item 8.4.3.2, SAE/Rel-8, which are incorporated by reference herein.
According to the above-mentioned documents, a new handover entity named Forward Attachment Function (FAF) is introduced. Generally, a FAF enables interworking for preparation of handover (in both directions) between WiMAX and 3GPP mobile access networks. A FAF acts as an E-UTRAN eNB towards the 3GPP EPC and RANs, acts as a WiMAX ASN towards WiMAX access networks (using WiMAX as an example for non-3GPP networks), and signals with the UE.
FIG. 1 exemplarily illustrates a FAF 100 interworking with 3GPP RANs (Radio Access Networks) 102 and a WiMAX access network 104 standing exemplarily for a non-3GPP access network. The FAF 100 connects via the S301 (also denoted as S1-MME) interface to the MME (Mobility Management Entity) of the EPC network in turn connected to the 3GPP mobile access networks GERAN, UTRAN, and E-UTRAN 102. Further, the FAF 100 is connected via the X101 interface to the WiMAX ASN (Access Service Network) of the WiMAX mobile access network 104. The FAF 100 acts as an E-UTRAN eNB (evolved Node B) towards the 3GPP EPC and RANs 102 over the S301 interface and as a WiMAX ASN towards WiMAX access network 104 over the X101 interface. The FAF 100 is further connected to a mobile terminal (UE 106) over the X200 (also denoted as LTE RRC) interface, e.g. via an IPsec (Internet Protocol security) tunnel T1 or T2.
When the UE 106 is using WiMAX access and intends to handover to one of the 3GPP-RANs 102, it performs a pre-attachment via the X200 reference point and the FAF 100 to the EPC nodes MME, SGSN (Serving GPRS Support Node (GPRS (General Packet Radio Service)), Serving GW (Serving Gateway), and PDN GW (Packet Data Network Gateway), as if it was attached via an E-UTRAN, wherein the FAF 100 mimicks an eNB. This enables that the handover can be optimized (i.e. accelerated), as context related to the usage of 3GPP access networks for the UE 106 is already established in the EPC nodes.
In the other direction, when the UE 106 is connected to GERAN, UTRAN or E-UTRAN 102 and intends to handover to WiMAX 104, the UE 106 can via the FAF 100 and via the X200 and X101 reference points already reserve resources in the WiMAX network 104.
The above-referenced documents do not solve the problem of how a UE may discover a FAF in a situation of a possible handover between 3GPP access network and non-3GPP access network. Consider for example the case of a possible handover to WiMAX. In this case it is desirable that the UE does only connect to the FAF when a handover to WiMAX is in fact feasible, i.e. when for example the UE is within the coverage of a WiMAX cell and/or the WiMAX access has high priority according to the access network selection policies of the UE, etc. In case a handover is not feasible, too much signaling occurs and state would be established unnecessarily in the FAF for the UE.
It is known to provide neighbour cell information in 3GPP access radio cells for neighbouring 3GPP cells, but not for neighbouring or overlapping non-3GPP cells. One way to address the problems above would be to extend 3GPP RAN information provisioning mechanisms to UEs such that the base stations/radio network controllers can provide neighbour cell information about neighbouring non-3GPP access cells (e.g. WiMAX) to the UEs, so that the UE knows when to connect to the FAF. However, this would require modifications to the 3GPP access networks and would lead to an increasing transmission overhead due to the new neighbour cell information.
A similar problem occurs in the complementary case of a possible handover to a 3GPP access network from a non-3GPP access network. For an optimized handover, the setup of a connection to the FAF—while the UE is connected to, e.g., WiMAX—would require that “suspended” 3GPP bearers are established in the evolved 3GPP core network for all active data sessions; when the WiMAX-to-3GPP-access handover occurs the data sessions active via WiMAX are then redirected to the already established “suspended” 3GPP bearers. The setup of the “suspended” 3GPP bearers requires signaling between 3GPP core network entities such as, e.g., the FAF, MME, Serving GW, PDG GW, HSS (Home Subscriber Server), as well as the usage of system resources in the core network nodes for maintaining the corresponding 3GPP bearer states. It is therefore desirable that the connection between the UE and the FAF as well as the subsequent core network signaling and bearer establishment/maintenance is only triggered when a WiMAX-to-3GPP-access handover is imminent.
The S2-075625 document only mentions that the UE may discover neighbour 3GPP cells while operating on mobile WiMAX access, and that this may be achieved with background scanning (e.g. when the UE has a dual-receive configuration) or by receiving the neighbour cell information from the network, e.g. via a user-plane mechanism or via a cell broadcast mechanism. However such an implementation would come along with the problems already discussed above.